Harvard: Marijuana Doesn’t Cause Schizophrenia

Good news for people who’ve worried that smoking too much marijuana (cannabis) — especially as a teenager — might lead to some dramatic problems in the future, even schizophrenia.

New research from Harvard Medical School, in a comparison between families with a history of schizophrenia and those without, finds little support for marijuana use as a cause of schizophrenia.

“The results of the current study suggest that having an increased familial morbid risk for schizophrenia may be the underlying basis for schizophrenia in cannabis users and not cannabis use by itself,” note the researchers.

The new study is the first family study that, according to the researchers, “examines both non-psychotic cannabis users and non-cannabis user controls as two additional independent samples, enabling the examination of whether the risk for schizophrenia is increased in family members of cannabis users who develop schizophrenia compared with cannabis users who do not and also whether that morbid risk is similar or different from that in family members of schizophrenia patients who never used cannabis.”

Marijuana use is becoming increasingly commonplace as two U.S. states have already legalized its use next to alcohol for adults. Some previous studies have suggested that there may be a correlational link between teenage marijuana use and the increased likelihood of being diagnosed with schizophrenia in the future.

So researchers from Harvard Medical School and the VA Boston Healthcare system got together to determine whether family risk for schizophrenia is a crucial factor underlying the association between the development of schizophrenia in teens who smoke marijuana.

The researchers recruited 282 subjects from the New York and Boston metropolitan areas who were divided into four groups: controls with no lifetime history of psychotic illness, cannabis, or any other drug use; controls with no lifetime history of psychotic illness, and a history of heavy cannabis use during adolescence, but no other drug use; patients with no lifetime history of cannabis use or any other drug and less than 10 years of being ill; patients with a history of heavy cannabis use and no other drug use during adolescence and prior to the onset of psychosis.

Information about all first-, second-, and third-degree relatives was obtained, as well as information about any other relative who had a known psychiatric illness. This resulted in information on 1,168 first-degree relatives and a total of 4,291 relatives. The study gathered together information regarding cannabis use, and family history regarding schizophrenia, bipolar disorder, depression and drug abuse.

The researchers concluded that the results of the current study, “both when analyzed using morbid risk and family frequency calculations, suggest that having an increased familial risk for schizophrenia is the underlying basis for schizophrenia in these samples — not the cannabis use.

“While cannabis may have an effect on the age of onset of schizophrenia it is unlikely to be the cause of illness,” said the researchers, who were led by Ashley C. Proal from Harvard Medical School.

“In general, we found a tendency for depression and bipolar disorder to be increased in the relatives of cannabis users in both the patient and control samples. This might suggest that cannabis users are more prone to affective disorders than their non-using samples or vice versa.” Future research is needed to understand this relationship.

Drug abuse also appears to have an important genetic component.

“Drug abuse is present more frequently in family members of all 3 samples compared to those of non-cannabis abusing controls. This is in line with past research confirming a genetic predisposition for drug use.”

The research was published earlier this month in Schizophrenia Research.

Source: Schizophrenia Research

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John M. Grohol, Psy.D.

Dr. John Grohol is the founder & CEO of Psych Central. He is an author, researcher and expert in mental health online, and has been writing about online behavior, mental health and psychology issues -- as well as the intersection of technology and human behavior -- since 1992. Dr. Grohol sits on the editorial board of the journal Cyberpsychology, Behavior and Social Networking and is a founding board member and treasurer of the Society for Participatory Medicine. You can read his full bio here.